Studies have shown that purine-pyrimidine alternating sequences, such as (C-G)n.(C-G)n and (T-G)n.(C-A)n, can form a left-handed helical conformation called Z-DNA. We previously found indications for the existence of such potential Z-DNA-forming sequences in natural genomes by hybridization. Here, we have comfirmed our previous observation by direct DNA sequencing. The human DNA library was screened by hybridization with poly d(T-G).poly d(C-A). Among many positive clones, five were randomly isolated and characterized in detail. DNA sequencing revealed that four clones contain a precisely alternating (T-G)n (n = 20,15,10,10), while the remaining one has a (T-G)15, with four bases out of alternation. The sequence data also revealed the characteristics of (T-G)n sequence with respect to its structure and organization in the human genome. The same strategy was applied directly to show the presence of (C-G)n sequence in the genome. However, such a sequence has not been isolated from the mouse DNA library, probably because the hybridization procedure does not recognize a (C-G)n sequence. These results have confirmed the general and abundant occurrence of (T-G)n, a potential Z-DNA-forming sequence. Attempts are now underway to establish an assay system to detect Z-DNA in biological samples, and to test the hypothesis that Z-DNA may play a crucial role in the regulation of gene expression and in carcinogenesis.